Printed circuit boards (PCB) have long been used as the base for sophisticated electronic systems. An electrically insulating sheet, originally phenolic impregnated fabrics and now generally fiberglass reinforced resins, is coated with copper cladding and has appropriate patterns etched into the cladding. In years past, most electronic components had wire leads that extended through holes drilled into the cladding pattern and filled with solder to make the required ohmic connections. More recently, surface bonding of relatively short leads to the cladding has become common, allowing for high-speed robotic placement of components thus increasing manufacturing productivity. PCBs typically have components on one side with various height profiles and require even support while manufacturing processes are done to the opposite side such as screen printing and pick-and-place operations.
Industry manufacturing processes for printed circuit boards generally provide that the PCB or substrate be held in place by constructing a vacuum box whose edges correspond to the perimeter edges of the substrate, whereupon the substrate becomes the top lid of the box and is held down or in place by a vacuum. Substrates are typically made of phenolic impregnated fabrics and more recently, fiberglass reinforced resins. Under various manufacturing operations the substrates can flex and frequently require underside support within the constructed vacuum box so that precise placement of components or cladding can be performed.
A vacuum box is commonly used in PCB manufacturing for screen-printing of circuitry or other operations such as component placement (i.e., pick-and-place). The vacuum box is generally an adjustable device that is used to support the PCB during manufacturing. One exemplary vacuum box is manufactured by MPM Corporation and has been used very successfully by the electronics assembly industry, specifically the surface mount technology industry. There are likely over 5000 such vacuum box systems world wide that are used as a means of holding the PCB substrate in place during the screen printing of solder paste onto the PCB substrate and/or component placement. The general configuration and use of a standard vacuum box is described in greater detail below with reference to FIGS. 1a through 7.
The vacuum box 100 is designed to accommodate PCB substrates from about 1.5″ to 15.0″ wide and up to lengths of 14.5″. Referring to FIGS. 1a and 1b, the vacuum box 100 has a table 1 that forms the bottom of the vacuum box 100 with vacuum holes 2 in the table that apply a vacuum capable of retaining the PCB using a large volume of air flow. Under the table 1 is a plenum (not shown) that surrounds the holes in the table 1 and accommodates the ductwork that couples the table 1 with the vacuum source. The table 1 supports a fixed front rail 3 (See also FIGS. 2a and 2b) and a moveable rear rail 4 (See also FIGS. 3a and 3b) that make up two opposing sides (i.e., front and rear) of the box. On each outer side of the front and rear rails 3 and 4 is the conveyor system 5 with conveyor belt 5a, typically placed as close to the front and rear rails as possible, for transporting the PCBs to and from the vacuum box 100.
The front rail 3 (see FIGS. 2a and 2b) is fixed to table 1 by mechanical fasteners 9 and is typically 17″ long. At the top of the front rail 3 is a thin blade 6 upon which one edge of the PCB substrate sits, typically along the length of the PCB substrate, and is parallel to the direction of travel for the conveyor system 5.
The rear rail 4 (see FIGS. 3a and 3b) is moveable along slot 7 (see FIG. 1a) in the table 1 and is also 17″ long. The slots 7 are usually configured as T-Slots that are configured to receive a mating T-Bolt or threaded fastener for slideably coupling two or more members (i.e., movable rear rail 4 with table 1). The T-Slots 7 of FIG. 1a formed table 1 engage T-Slot nuts and bolts 9, which engage mounting, brackets in the rear rail 4 (see FIG. 3a). In use, the T-Slot nuts 9 are loosened, whereupon the rear rail 4 can be moved and adjusted to the width of the PCB substrate and then tightened to fix the position of the rear rail 4 with respect to the front rail 3.
Similar to the front rail 3, at the top of rear rail 4, is another thin blade 8 upon which the opposite edge of the PCB substrate engages. The thin blade 8 is parallel to the direction of the conveyor system 5 upon which the PCB substrate traverses. The PCB substrate then straddles the two rails (front rail 3 and rear rail 4) with the thin blades 6, 8 supporting the PCB substrate along the length of the PCB edges. It is common for the edges of the PCB to actually protrude past the front and rear edges established by the thin blades 6 and 8 by about 0.100″ on each side so that the PCB can engage and ride along the conveyor (See FIG. 6).
The front and rear rails 3 and 4 have clamping rails 10 which engage side plates 11 (See FIGS. 1a, 1b and 4) by way of a clamping block 12. The clamping rail 10 on the front rail 3 is located higher than the clamping rail 10 on the rear rail 4 (see FIGS. 2a and 3a), which in turn requires that four unique side plate configurations be used. The side plates are generally designated front and rear as well as left and right side plates. The industry commonly utilizes side plate nomenclature to designate the operative position of the side plates and this nomenclature is marked on the side plates via stamping or other known marking methods (e.g., screen printing, engraving, etching etc.). Correspondingly, the side plates are marked according to the intended installation position (e.g., LF-left front 11a, LR-left rear 11b, RF-right front 11c, and RR-right rear 11d). The four side plates 11a-d make up the other two opposing sides of the vacuum box 100, with the PCB substrate forming the top or lid. The side plates typically have notches 30 cut into them so that each side plate 11a-d can be drawn as close to the opposite rail (front or rear rails 3 and 4) and thus the notch 30 receives the clamping rail 10 of the opposing front or rear rails 3 and 4. As a result, the side plates 11a-d may be positioned at a minimum separation distance from each other and thus accept a narrow PCB.
The side plates 11a-d (see FIGS. 1a-c and 4-6) have one surface that is generally smooth/flat (i.e., devoid of protrusions on at least one side surface). The front and rear side plates for the vacuum box 100 (e.g., left or right side) are positioned in sliding relationship with respect to each other, see FIGS. 1a-c. The pair of front and rear side plates may then be adjusted to the width of the PCB substrate and thus dimensionally define the distance between the front and rear rails 3 and 4. Additionally, the pair of coupled side plates can be positioned at various distances along the front and rear rails which in turn establishes the length of the PCB substrate. In summary, the two pair of side plates are in sliding relationship with each other as well as adjustable along the length of the front and rear rails which in turn forms a box that is adjustable in both width and length to accommodate various sizes of printed circuit boards.
Referring to FIG. 4, on the opposite side of the side plate 11a is a clamping block 12 that interlocks with the clamping rails 10 of either front or rear rails 3 and 4. The clamping block 12 is securely attached to the side plate 11 usually by screws, pins or combinations of both. A screw 13 at the top of clamping block 12 is tightened which drives a pin 14 into a clamp lip 15, which in turn displaces a clamp lip 15 such that the clamp lip 15 engages a rear notch surface 16 of the corresponding clamp rail 10. The interaction and physical engagement between the clamp lip 15 of the clamping block 12 and rear notch surface 16 of the clamping rail 10, secures the clamping block 12 and side plate 11 in position with respect to the front and rear rails 3 and 4. Reference should be made to FIGS. 5 and 6, which illustrate how the side plates/clamping blocks engage the clamping rail.
Generally, the side plates 11a-d come in several sizes; extra-small, small, medium, large and extra-large that accommodate the various widths of any given PCB substrate up to 15.00″ wide. Each size has a corresponding front-left, rear-left, front-right and rear-right side plate and is designated as discussed above. The side plate sizes typically overlap slightly with the next larger size thus allowing for a continuum of PCB widths from the minimum size up to a maximum that may be accommodated within the vacuum box 100. All of the mounting hardware for each size side plate is generally similar. For example, the clamping block 12, screw 13, pin 14 and clamping lip 15 are interchangeable with the various side plates 11. Additionally, it is understood that front-left and front-right side plates are mirror images of each other and the same is true for the rear-left and rear-right side plates.
The side plates 11a-d have a clamping block 12 usually made of stainless steel that engages the clamping rails 10 on the front and rear rails 3 and 4. Correspondingly, the side plates 11a-d may slide along the clamping rail 10 thus allowing the side plates 11a-d to be adjusted to the length of any PCB substrate. The side plates 11 are configured to support the PCB substrate along its width at the front and rear edges of the PCB substrate (with respect to the direction of travel along the conveyor system for the PCB). Additionally, there is little-to-no overhang of the PCB in relation to the side plates 11a-d. 
After the front and rear rails 3 and 4 are set and the four side plates 11a-d are clamped down using clamping blocks 12, they form four sides of the vacuum box 100 and support the PCB 33 substrate around its perimeter. The table 1 forms the bottom of the box and the PCB substrate itself forms the lid or top of the vacuum box. Vacuum is then pulled through the holes 2 in the table 1 and suction holds the PCB 33 substrate down against the thin blades 6 and 8 of the front and rear rails 3 and 4 and the side plates 11a-d. The vacuum box 100 essentially forms a cube with, theoretically, no holes or substantial air leaks. However, any given PCB substrate can have numerous holes and slots in them and the vacuum box itself is not airtight. In this regard, to operatively retain the PCB, a large volume of air is drawn in as vacuum to compensate for any leakage in the vacuum box/PCB assembly. Consequently, it is desirable to keep vacuum leakage to a minimum for efficiency concerns.
In operation (see FIG. 6), the table 1, front and rear rails 3 and 4 including side plates 11a-d move up and down to engage the PCB 33. At a predetermined stop point the conveyor 5 with the PCB 33 translating there along, will stop and the vacuum box 100 (the front and rear rails and side plates 11a-d, including the table 1) is raised to engage the PCB 33, lifting the PCB 33 off of the conveyor rails and belts 5, 5a. An upper fixture (not shown) holding a stencil frame 101 and stencil 102 is then lowered onto the PCB 33. Cameras 104, in conjunction with an assembly line computer system, aligns the apertures in the stencil 102 with a corresponding target or “land” pattern on the PCB 33. After alignment, solderpaste is then squeegeed onto the PCB 33 and the stencil 102 is lifted away, leaving a corresponding amount of solderpaste on the PCB. The table 1 and vacuum box 100 assembly then lowers, placing the edges of the PCB 33 (lengthwise) back onto the conveyor belts 5a. The PCB is then shuttled to the next station (typically for pick-and-placement of components), and a new PCB 33 moves into the printer station whereupon the procedure is repeated.
Today, electronic devices are increasingly miniaturized and it has become desirable to mount components on both sides of a PCB. However, there are a number of problems associated with installing parts on the second side after components have been mounted on the first side. For example, the board cannot be held flat with downwardly projecting components of various sizes and thicknesses mounted on the lower side. This problem is most acute when solder paste is to be printed on the second side. During manufacturing, the PCB is required to be held flat and level so that paste application and component placement can be accurately performed upon the second side. This solder paste application and component placement is very difficult if the PCB is not properly secured and held in place.
In the past, several attempts have been made to provide PCB support within the vacuum box so that screen-printing and component placement may be performed. Firstly, in high production run circumstances, aluminum plates or similar materials have been machined in a pattern corresponding to the topography of the first side of the PCB so that components protruding from the first side may pass-through the plate. As a result, the aluminum plate then provides structural support to the PCB during the manufacturing process. This approach is not practical for manufactures or subcontractors producing a limited quantity of printed circuit boards or for boards of vastly different configurations because each PCB design/configuration would require a custom support plate which is expensive and time consuming to machine.
Secondly, internal vacuum box supports have been made by casting a plaster-like material into a mold corresponding to a particular PCB to form a support having pockets for receiving the components on the downwardly extending board side. While effective where a large number of identical boards are to be manufactured, this method is not cost effective where only a few boards are to be made or where custom boards are being manufactured.
Thirdly, during set up of the vacuum box a series of fixed PCB substrate supports have been used that fit between the table surface and the PCB. These supports are usually held to the table by magnets. These supports are typically either pins or blades. The pins are usually about 0.125″ to 0.250″ in diameter and correspond to the height of the front, rear and side plates and are secured using a magnetic base. The blades are typically 0.050″ to 0.0625″ thick and up to 2.00″ wide and are the same height as the rails and side plates and have a magnetic base. These fixed height supports must be positioned to engage the PCB substrate where no components have been installed otherwise damage to the components may occur during screen-printing or component placement. Also, support can be uneven, and where PCB substrates are populated with a high density of components, it may be very difficult to locate an area that is unpopulated with components.
Fourthly, a series of gel packs 19 were implemented to provide PCB support within the vacuum box, see FIG. 7. The gel packs are essentially packets of gel or other deformable medium that are positioned inside the vacuum box, which conforms to the underside topology of the PCB during printing or component placement. This solution, although somewhat effective, has been less than successful, in that the vacuum box requires the gel packets 19 to be individually constrained using independent mounts 20 to allow vacuum to pass between the individual packets.
Moreover, the gel packs 19 can accommodate component topologies of only limited height. Furthermore, because the gel packets 19 have limited compressibility, they may push the PCB substrate up off the edge(s) of one or more sides of the vacuum box, breaking the vacuum seal and possibly causing misalignment of the PCB registration to the stencil apertures. This is particularly true if a gel packet is under a high profile part, which requires substantial displacement of the gel. In this situation the gel packet 19 would then have to be removed from this position, leaving the PCB substrate unsupported in this area. Conversely, when pressure is applied to the stencil/PCB as the squeegee passes over the substrate, the gel packs 19 may compress and only provide limited support and thus permit the PCB to deflect.
Additionally, the spacing 21 between the gel packets should be optimized such that the majority of the vacuum holes in the table are able to apply vacuum to the PCB. The deficiencies with the gel pack solution can become significant, especially if the PCB substrate is very thin.
Finally, another attempt to provide internal PCB support within the vacuum box uses various substrate support devices placed within the vacuum box, where the support devices provide a plurality of deformable or adjustable pins that conform to the component topology of the PCB. However, this attempt to provide support using such substrate support devices requires that custom devices be manufactured or modified that correspond to the length and/or width of the PCB. Thus, customizing each substrate support device to fit within the perimeter of the front and rear rails 3 and 4 and side plates 11a-d is not cost effective, time efficient or practical unless a manufacturer is only making one size PCB, which is unlikely in the present electronics manufacturing environment. This solution lacks the flexibility that PCB manufacturers need to produce and assemble many different sizes of PCB substrates.
Efforts to provide internal vacuum box support for a printed circuit board, having components mounted on one side while additional components are installed or operations performed on the opposite side, have not met with much success to date.